1. Field of the Invention
The present invention relates to a diversity receiver that is provided with two antenna systems and an antenna switch for switching between these two antenna systems, and more particularly to an antenna switching control method in a diversity receiver for implementing control to switch to, of the antennas of the two systems, the antenna having the better reception state.
2. Description of the Related Art
In recent years, the CDMA (Code Division Multiple Access) communication method, which can overcome interference and obstructions, has received attention as a communication method for use in mobile communication systems.
In addition, a diversity technology, which is an antenna switching method for switching between antennas of two systems, has been employed as a means for improving reception performance in a mobile communication system in which the location of a communication partner changes. In antenna diversity receivers of the prior art in which this diversity technology is applied in a mobile terminal of the above-described CDMA communication method, several problems have been encountered as described below.
In the TDMA (Time Division Multiple Access) method, time multiplexing allows a receiver to take advantage of vacant reception time intervals to switch between two antennas and select the antenna having better reception quality. In contrast, a receiver in a system such as the CDMA method, in which time division multiplexing is not performed, must constantly receive data throughout communication.
As a result, two methods have been adopted: one using two sets of antennas, radio receivers, and despreaders in which data that have been received by the two antennas are RAKE synthesized, and another having only two antennas in which reception switches between the two antennas even during communication to measure the reception level in the other antenna.
The first method, while providing good reception performance, is disadvantageous in that it requires two receiving systems and therefore entails increased circuit scale and current consumption. The second method requires only two antennas and one receiving system and therefore offers the advantages of small circuit scale and low current consumption. However, the second method necessitates switching between antennas during continuous reception and monitoring the level of the other antenna, and therefore has the disadvantages of deterioration of reception quality during switching and the potential for losing data. Although adjustment of the monitoring period can suppress data loss of voice data or packet data to within the permissible quality range, this solution is not preferable because the loss of communication control data, which is communication protocol information, can lead to handover delays or line disconnections.
A diversity receiver is proposed in Japanese Patent Laid-Open Publication No. H11-55168 that enables switching between antennas without deterioration in the reception characteristics by switching to the antenna having the higher received electric field intensity in time intervals in which reception of OFDM (Orthogonal Frequency Division Multiplex) signals is not necessary.
When a diversity reception technology is thus used in a communication method having time intervals in which reception is not necessary, a diversity receiver can be achieved that switches antennas without a deterioration of reception characteristics if antennas are switched in these time intervals in which reception is not necessary.
However, as noted in the foregoing explanation, in the case of a communication method that requires continuous reception such as the CDMA communication method, time intervals that are not required for reception basically do not exist, and the simple application of diversity reception technology therefore inevitably results in deterioration of reception characteristics.
The diversity receiver that is disclosed in, for example, Japanese Patent Laid-Open Publication No. 2002-300096 applies diversity transmission technology to a CDMA receiver. This diversity receiver of the prior art, however, involves the switching of transmission antennas by determining the state in which the average transmission power is at a minimum to be the best communication state, and does not involve switching of reception antennas, and therefore does not realize a diversity receiver that can switch between antennas without suffering deterioration in reception characteristics.